This invention relates to circuitry for supplying drive power to any of a number of paths. More particularly, it is concerned with drive circuitry including arrangements for determining if the proper number of paths are being driven.
Crosspoint switching arrays employing solid state devices have been developed for use in switching networks of communication systems. An array of switching circuits employing solid state devices is described and claimed in U.S. Pat. No. 3,826,873 entitled "Switching Circuits Employing Latching Type Semiconductor Devices and Associated Control Transistors" which issued on July 30, 1974 to A. Frederick Susi and is assigned to the assignee of the present invention. As described in the patent to Susi each switching circuit of an array is connected to one control line in each of two sets of control lines. An individual switching circuit is activated to permit the transmission of signals by the simultaneous application of momentary pulses of opposite polarity on its two control lines. The two sets of control lines, one for positive-going control pulses and the other for negative-going control pulses, are connected to the switching circuits of the array so that only one switching circuit is selected when a positive-going pulse is applied to one of the control lines of one set and a negative-going pulse is applied to one of the control lines of the other set.
In switching arrays of the foregoing type it is desirable before activating a switching circuit during a switching operation to ascertain that one and only one switching circuit is being selected. That is, it is desirable to be able to determine that a control pulse will be present on one and only one positive-going control line and also that a control pulse will be present on one and only one negative-going control line. Thus it is assured that one and only one switching circuit of the array will be activated during the switching operation to permit the transmission of signals therethrough.
Suitable drive circuitry for use with switching circuits which includes an error detection arrangement for providing an error signal if a control pulse is present on more or less than one of the control lines of each set is described in detail in application Ser. No. 520,413 filed Nov. 4, 1974, now U.S. Pat. No, 3,940,571 issued Feb. 24, 1976, entitled "Drive Circuitry with Error Detection" by Albert H. Ashley and assigned to the assignee of the present application. The circuitry described in the application employs a drive circuit in each control line for producing a control pulse. A resistance network including a resistance connected to the output of each drive circuit produces a voltage at a summing point which is proportional to the number of control lines of a set having control pulses present. The summing point is connected to logic circuitry including comparators. The logic circuitry produces an error signal if none of the control lines of the set has a control pulse present or if more than one of the control lines of the set have control pulses present.
The drive circuitry described in the above-mentioned application has been found to operate satisfactorily for systems having of the order of eight control lines in a set as disclosed in the application. Although the number of control lines of a set may be increased, under certain conditions if the number of lines of a set is greatly increased, for example to twenty lines, minor variations in the characteristics of components may introduce errors. More specifically, variations in the output voltages of the drive circuits affect the voltage present at the summing point. Since the increment of voltage produced at the summing point by each drive circuit producing a pulse is relatively small for a relatively large number of control lines, the possibility of error increases with increased number of control lines.